1. Field of the Invention
The present invention relates to a configuration of thin-film transistors (TFTs) formed on a flexible substrate (i.e., having mechanical flexibility) such as a resinous substrate which can be made of engineering plastics. The invention also relates to a method of fabricating such thin-film transistors. Furthermore, the invention relates to an active matrix liquid crystal display fabricated, using these thin-film transistors.
2. Prior Art
Thin-film transistors formed on glass substrates or on quartz substrates are known. Thin-film transistors formed on glass substrates are chiefly used in active matrix liquid crystal displays. Since active matrix liquid crystal displays can display images with high response and with high information content, it is expected that they can supplant simple matrix liquid crystal displays.
In an active matrix liquid crystal display, one or more thin-film transistors are disposed as a switching element at each pixel. Electric charge going in and out of the pixel electrode is controlled by this thin-film transistor. The substrates are made of glass or quartz, because it is necessary that visible light pass through the liquid crystal display.
Liquid crystal displays are display means which are expected to find quite extensive application. For example, they are expected to be used as display means for card-type computers, portable computers, and portable electronic devices for various telecommunication appliances. As more sophisticated information is treated, more sophisticated information is required to be displayed on the display means used for these portable electronic devices. For example, there is a demand for functions of displaying higher information content and moving pictures as well as numerals and symbols.
Where a liquid crystal display is required to have a function of displaying higher information content and moving pictures, it is necessary to utilize an active matrix liquid crystal display. However, where substrates made of glass or quartz are used, various problems take place: (1) limitations are imposed on thinning of the liquid crystal display itself; (2) the weight is increased; (3) if the thickness is reduced in an attempt to reduce the weight, the substrate breaks; and (4) the substrate lacks flexibility.
Especially, card-type electronic devices are required to be so flexible that they are not damaged if slight stress is exerted on them when they are treated. Therefore, liquid crystal displays incorporated in these electronic devices are similarly required to be flexible.
The invention disclosed herein provides an active matrix liquid crystal display having flexibility.
One available method of imparting flexibility to a liquid crystal display is to use plastic or resinous substrates which transmit light. However, because of poor heat resistance of resinous substrates, it is technically difficult to form thin-film transistors on them.
Accordingly, the invention disclosed herein solves the foregoing difficulty by adopting the following configuration:
One invention disclosed herein comprises: a filmy resinous substrate; a resinous layer formed on a surface of said resinous substrate; and thin-film transistors formed on said resinous layer.
A specific example of the above-described configuration is shown in FIG. 1. In the configuration shown in FIG. 1, a resinous layer 102 is in contact with a PET film 101 having a thickness of 100 xcexcm, the PET film being a filmy resinous substrate. Inverted-staggered thin-film transistors are formed on the resinous layer.
The material of the filmy resinous substrate can be selected from PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PES (polyethylene sulfite), and polyimide. The requirements are flexibility and transparency. Preferably, the maximum temperature that the material can withstand is made as high as possible. If the heating temperature is elevated above 200xc2x0 C., oligomers (polymers having diameters of about 1 xcexcm) are generally deposited on the surface, or gases are produced. Therefore, it is quite difficult to form a semiconductor layer on the resinous substrate. Consequently, the material should have the highest possible processing temperature.
In the above-described structure, the resinous layer acts to planarize the surface of the resinous substrate. The planarization also serves to prevent precipitation of oligomers on the surface of the resinous substrate during steps involving heating such as the step for forming the semiconductor layer.
The material of this resinous layer can be selected from methyl esters of acrylic acid, ethyl esters of acrylic acid, butyl esters of acrylic acid, and 2-ethylhexyl esters of acrylic acid. Even if resinous substrates are used, this resinous layer can suppress the drawbacks with fabrication of the afore-mentioned thin-film transistors.
The configuration of another invention comprises the steps of: forming a resinous layer on a filmy resinous substrate; forming a semiconductor layer on said resinous layer by plasma-assisted CVD; and forming thin-film transistors, using said semiconductor layer.
The configuration of a further invention comprises the steps of: heat-treating a filmy resinous substrate at a given temperature to degas said resinous substrate; forming a resinous layer on the filmy resinous substrate; forming a semiconductor layer on said resinous substrate by plasma-assisted CVD; and forming thin-film transistors, using said semiconductor layer.
In the above-described structure, heat-treatment is made to degas the resinous substrate, in order to prevent escape of gases from the resinous substrate during later processes involving heating. For example, if gases are released from the resinous substrate when a semiconductor thin film is being formed on the resinous substrate, then large pinholes are formed in the semiconductor thin film. This greatly impairs the electrical characteristics. Accordingly, the substrate is heat-treated at a temperature higher than heating temperatures used in the later processes, to degas the resinous substrate. In this way, release of gases from the resinous substrate during the later steps can be suppressed.
The configuration of a yet other invention comprises the steps of: heat-treating a filmy resinous substrate at a given temperature; forming a resinous layer on said filmy resinous substrate; forming a semiconductor layer on said resinous substrate by plasma-assisted CVD while heating the substrate to a temperature lower than said given temperature; and forming thin-film transistors, using said semiconductor layer.
The configuration of a still other invention comprises the steps of: heat-treating a filmy resinous substrate at a given temperature which is higher than any heat-treatment temperature used in other steps; forming a resinous layer on said filmy resinous substrate; forming a semiconductor layer on said resinous substrate by plasma-assisted CVD; and forming thin-film transistors, using said semiconductor layer.
The configuration of a still further invention comprises: a pair of filmy resinous substrates; a liquid crystal material held between said resinous substrates; pixel electrodes formed on a surface of at least one of said resinous substrates; thin-film transistors connected with said pixel electrodes and formed on said resinous substrate; and resinous layers formed on surfaces of said filmy resinous substrates to planarize the surfaces.
A specific example of the above-described structure is shown in FIG. 3. In the structure shown in FIG. 3, a pair of resinous substrates 301, 302, a liquid crystal material 309 held between these resinous substrates, pixel electrodes 306, thin-film transistors (TFTs) 305 connected with the pixel electrodes 306, and a resinous layer 303 for planarizing the surface of the resinous substrate 301.